Why not do the mod instead of buying and worrying about lugging around all of the extra equipment? Those are the kinds of little things that always get left behind when you want to take your theremin out of the house to play.

While you're at it, why not just ADD the 47K resister beside the 4.7K plus a single-pole/double-throw (SPDT) switch so you can switch between them as needed? This type of switch (HI/LOW Output) is common on most vintage synths and keyboards.

The possible "problem" is that guitar and bass amps have a non-linear frequency response in order to compensate the response of magnetic pickups which have much less output voltage at lower frequencies. So these amps act as a low-pass filter which risks to dull the theremin's sound...

Thanks, Thierry, I forgot to mention that (frequency response tailoring)on my post to that other place that shall not be named :)

Guitar amps sometimes have a "bright" switch. That circuit switches in a treble boost filter that attempts to compensate for the roll-off from the guitar. Magnetic guitar pickups actually respond to the rate-of-change-of-flux rather than linearly responding to string vibration frequencies. This, in essence, amounts to a low-pass filter response.

But the "dull" sound probably has more to do with running the guitar amp at reduced volume (or attenuated input signal) then anything else. This effect is explained in reference sources as "loudness compensation" and those usually refer you to the "Fletcher-Munson" curves. Those graphs show the ear's frequency response at various volume levels.

Another interesting thing that can be seen from those graphs is the peak around 2-3kHz. In guitar amps and in some early hi-fi amps, they had a "presence" control which allowed you to boost the frequency response in that range. Boosting response around that point has the effect of "bringing a singer to the front".